Promoters
The genetic enhancement of plants and seeds provides significant benefits to society. For example, plants and seeds may be enhanced to have desirable agricultural, biosynthetic, commercial, chemical, insecticidal, industrial, nutritional, or pharmaceutical properties. Despite the availability of many molecular tools, however, the genetic modification of plants and seeds is often constrained by an insufficient or poorly localized expression of the engineered transgene.
Many intracellular processes may impact overall transgene expression, including transcription, translation, protein assembly and folding, methylation, phosphorylation, transport, and proteolysis. Intervention in one or more of these processes can increase the amount of transgene expression in genetically engineered plants and seeds. For example, raising the steady-state level of mRNA in the cytosol often yields an increased accumulation of transgene expression. Many factors may contribute to increasing the steady-state level of an mRNA in the cytosol, including the rate of transcription, promoter strength and other regulatory features of the promoter, efficiency of mRNA processing, and the overall stability of the mRNA.
Among these factors, the promoter plays a central role. Along the promoter, the transcription machinery is assembled and transcription is initiated. This early step is often rate-limiting relative to subsequent stages of protein production. Transcription initiation at the promoter may be regulated in several ways. For example, a promoter may be induced by the presence of a particular compound or external stimuli, express a gene only in a specific tissue, express a gene during a specific stage of development, or constitutively express a gene. Thus, transcription of a transgene may be regulated by operably linking the coding sequence to promoters with different regulatory characteristics. Accordingly, regulatory elements such as promoters, play a pivotal role in enhancing the agronomic, pharmaceutical or nutritional value of crops.
At least two types of information are useful in predicting promoter regions within a genomic DNA sequence. First, promoters may be identified on the basis of their sequence “content,” such as transcription factor binding sites and various known promoter motifs. (Stormo, Genome Research 10: 394-397 (2000)). Such signals may be identified by computer programs that identify sites associated with promoters, such as TATA boxes, transcription factor (TF) binding sites, and CpG islands.
Second, promoters may be identified on the basis of their “location,” i.e. their proximity to a known or suspected coding sequence. (Stormo, Genome Research 10: 394-397 (2000)). Promoters are typically contained within a region of DNA extending approximately 150-1500 basepairs in the 5′ direction from the start codon of a coding sequence. Thus, promoter regions may be identified by locating the start codon of a coding sequence, and moving beyond the start codon in the 5′ direction to locate the promoter region.
Rice
Approximately half a billion tons of rice is produced each year world-wide. More than 90% of this rice is for human consumption (Goff, Curr. Opin. Plant Biol. 2:86-89 (1999)). Rice, however, is not only a commercially important crop; it is also a model for other cereal crops, such as sorghum, maize, barley and wheat.
Rice is a model crop for several reasons. First, the genes in rice are predicted to be generally arranged in the genome in an order that is similar to other cereal crops. In fact, comparisons of the physical and genetic maps of cereal genomes have suggested the existence of a colinearity of gene order among the various cereal genomes studied. (Goff, Curr. Opin. Plant Biol. 2:86-89 (1999)).
Second, studies of a number of individual genes indicate that there is considerable homology within gene families found among various cereal crops. This conservation of gene and protein sequences suggests that functional studies of genes or proteins from one cereal crop can help elucidate the function of similar genes or proteins in other cereal crops. Likewise, non-coding regulatory elements in rice, such as promoters, are predicted to display similar functions compared to related regulatory elements found in other cereal crops. Accordingly, a strong constitutive or tissue-specific promoters from one cereal is more likely to retain its function when introduced as a portion of a transgene into another cereal crop species (Goff, Curr. Opin. Plant Biol. 2:86-89 (1999).
Third, rice can be used as a model for other cereal genomes because its genome is smaller than those of other major cereals. The size of the rice genome is estimated at 420 to 450 megabase pairs. Sorghum, maize, barley and wheat have larger genomes (1000, 3000, 5000 and 16000 Mbp, respectively). Despite such differences in genome size, however, the number of genes in each of these crops is on the same order of magnitude. Thus, the smaller genome size of rice results in a higher gene density relative to the other cereals. Based on estimates of 30,000 genes in a cereal genome, rice will have on average one gene approximately every 15 Kbp. In contrast, maize and wheat have one gene approximately every 100 and 500 Kbp, respectively. This higher gene density makes rice an attractive target for cereal gene discovery efforts, genomic sequence analysis, and identification of regulatory elements, such as promoters (Goff, Curr. Opin. Plant Biol. 2:86-89 (1999)).
For these reasons, rice is a model for other crops. Accordingly, discoveries in rice may be extended to other crops. Thus, the identification of new genes, regulatory elements (e.g., promoters), etc. that function in rice is useful not only in developing enhanced varieties of rice, but also in developing enhanced varieties of other crops. In particular, developments in rice are applicable to other cereal crops, such as sorghum, maize, barley and wheat.
Clearly, there exists a need in the art for new regulatory elements, such as promoters, that are capable of expressing heterologous nucleic acid sequences in important crop species.